1. Field of the Invention
The present invention relates to a technique for adjusting a measurement device for measuring a distance to a tool holder in order to judge an abnormality of a state in which the tool holder is attached to a machine tool having a rotary shaft. More particularly, the present invention relates to a machine tool having a sensor which can be easily replaced when the above measurement device is automatically adjusted in a machine tool in which a tool holder such as a machining center, which will be referred to as an “MC” hereinafter, is used.
2. Description of the Related Art
An MC is a device in which various tools are automatically selected according to a machining process and the thus selected tool is automatically attached to a main spindle so as to conduct various types of machining. In this MC, tools are replaced by an automatic tool changing device (ATC). Using this ATC device, a tool holder, to which a tool is attached, is automatically taken out from a tool magazine and automatically attached to a main spindle.
FIG. 1 is a sectional view showing a state in which a tool holder 3 is attached to a main spindle 4. As shown in the drawing, the tool holder 3 has a conical engaging portion 3a, and this engaging portion 3a is engaged with a conical portion 4a to be engaged which is formed in the main spindle 4.
This procedure is described as follows. When a shaft rod 90 is pulled to the right, a ball holding body 91 and balls 92 are moved according to the movement of the shaft rod 90. When the balls 92 are moved, a pull stud (drawing bolt) 93 is pulled, and the conical engaging portion 3a of the tool holder 3 is pressed to the conical portion 4a, to be engaged, of the main spindle 4. Due to this pressing force, the engaging portion 3a is tightly contacted with the portion 4a to be engaged, and chucking is conducted.
FIGS. 2A and 2B are schematic illustrations showing a state in which the tool holder 3 for holding the tool 2 is attached to the main spindle 4. Usually, the tool holder 3 is properly attached in a state shown in FIG. 2A. However, when chips 94 are attached to this engaging portion as shown in FIG. 2B, the tool holder 2 is attached while the axis is being inclined. When machining is conducted in this state in which the axis is inclined, deviation is caused in the tool 2, and the machining accuracy of machining a workpiece is deteriorated.
A technique, by which it is detected whether or not chucking is properly conducted in the tool holder 3, is disclosed in the Japanese publications of unexamined patent applications of JP-A-2004-276145, 2004-42208, 2003-334742 and 2002-200542, which are applications made by the present applicant. According to these applications, a distance to the tool flange 3b of the tool holder 3 is measured by a displacement sensor such as an eddy current sensor, and the occurrence of an abnormality of a state in which the tool holder 3 is attached to the main spindle 4 is judged according to measurement data obtained in this measurement.